In the 20 years Peter Sutcliffe has been behind bars, he has struck up intense relationships with dozens of women. BBC News Online's Megan Lane asks what they see in the Yorkshire Ripper?

Peter Sutcliffe must appear to be the ideal pen pal to the string of women who write to him.

The convicted serial killer fills pages and pages with warm and chatty banter. "You are a breath of fresh air," he writes to one. To another: "I like this cloud nine thing with you."

There has always been a strong sexual attraction to men who are so-called 'bad boys'

Petruska Clarkson

Twenty years ago this month, Sutcliffe was sentenced to a minimum of 30 years for brutally murdering 13 women.

Despite his notoriety, he receives an average of 30 letters a week from women. Some are sympathetic, others lovelorn.

But there's a difference between writing to someone on death row, as many people do because they oppose capital punishment, and becoming obsessed with a dangerous stranger.

What possesses these women to forge a long-distance relationship with Sutcliffe? Three of his pen pals reveal all in an Everyman documentary entitled Dear Peter - Letters To The Yorkshire Ripper.

Diane Simpson exchanged letters and tapes

Diane Simpson, a handwriting analyst from Cheshire, has exchanged more than 500 letters and spent 400 hours visiting Sutcliffe in Broadmoor over the past 10 years.

She worked on the original manhunt and, still fascinated, wrote to him after his conviction. His letters piqued her interest by repeatedly hinting that he would confess to other crimes, she says.

Sandra Lester moved house to be near Sutcliffe

Artist Sandra Lester began writing to Sutcliffe in 1990 while trying to come to terms with the abuse she suffered as a child.

She read an article about him and decided to "extend a Christian hand of support". After exchanging letters daily for a year, she believed she had found lasting love with the killer.

But he refused to allow her visiting rights, telling prison authorities that he wanted numerous female friends.

Olive Curry refuses to accept Sutcliffe's denial

Olive Curry, from Tyneside, started writing to Sutcliffe because she believed he used to visit the canteen where she worked.

She says she wanted him to reveal the identity of his companion, whom she believed could have been his accomplice. Although Sutcliffe denies having been to the canteen, the pair have exchanged 500 letters.

'Sadistic thing to do'

Newcastle University psychologist Dr George Erdos says for men like Sutcliffe, letter-writing not only fills the long boring days behind bars.

Peter Sutcliffe: Reign of terror in late 1970s

"People who kill women, particularly prostitutes, do it for reasons of inadequacy.

"They don't like women, or they're frightened of them. Being in prison, an all-male environment, means there's little chance to vent that aggression.

"This way, he can manipulate women by telling them how special they are, then cause grief by saying, 'You know you're not the only one'. It's a sadistic thing to do."

'Bad boy' appeal

Professor Petruska Clarkson, a consultant psychologist and relationship psychotherapist, says convicts may also seek attention in this way because it's a basic human need to form bonds with others.

The Krays both married women who contacted them in prison

Both agree that it's harder to pin down what's in it for those on the outside.

Diane Simpson, Sandra Lester and Olive Curry have their reasons. Others may first make contact because they are lonely, curious, or caught up in a religious fervour to forgive the unforgivable, Dr Erdos says.

Professor Clarkson says some may fantasise that a man like Sutcliffe may be the way he is because he has yet to be loved by the right person - and they may well be the one.

"This is certainly a way to feel special and unique."

One of Sutcliffe's first letters to Diane Simpson

For someone who has been abused or neglected, it can be deeply satisfying to be special to someone so desperate for relationship.

"And there has always been a strong sexual attraction to men who are so-called 'bad boys'," Professor Clarkson says.

"Villains capture the imagination. Human beings are interested in those who live by extremes since they often do what other, more ordinary mortals, cannot bear to think of themselves capable [of doing]."